This subject just goes to show you how two people with similar experience can arrive at opposite conclusions. I too have shot hundreds of thousands of rivets, the overwhelming majority being AD5 and AD6 rivets, rivets that you have to LAY INTO. All were backed with a lump of common hardened steel. The idea of bucking itsy bitsy AD3 and AD4 rivets using tungsten seems laughable to me. These rivets are so small that most men can hand-squeeze them for criminy sakes. Of course vendors will always claim their product is vastly superior to the alternative and try mightily to convince us that giving them money is the answer to all our prayers. Now, having invested the sum of $90-$150, the tungsten purchaser is suddenly transformed into a true believer. My company known to spend taxpayer money with ease and breathless abandon...never thought to equip its production workers with tungsten bucking bars at least up until the time I quit. Not all that long ago, all company bucking bars were equipped with a radioactive plug buried deep inside. Its purpose was easy detection in case one was accidentally closed up in an assembly. That practice was stopped some time ago when all bucking bars were turned in and replaced with common hardened steel. I don't think there are a whole lot of F-4's, F-15's and F/A-18's that ever felt the strange kiss of tungsten upon their rivet shop heads.
Vastly more important than the material the bucking bar is made from is SIZE and SHAPE. These things and these things alone dictate how much utility you can reasonably anticipate from any given bucking bar be it hardened steel or tungsten. If you can't reach the work, that bar is useless.
Whether it is an AD3, AD4, AD5, or AD6 rivet, my favorite bucking bar, the bar I automatically reach for 90%-95% of the time is this one. This bar was used to buck every single AD6 rivet in my RV-6A wing spars. Pricey tungsten? Please. I've got other priorities to spend my money on.
Bottom line: If I thought tungsten might give me a 10%-30% mechanical advantage over its common steel cousin, I would expect its price to reflect that modest advantage with a 10%-30% premium price, not be priced like man jewelry.